Sealing structure and method of sealing electronic component

ABSTRACT

A sealing structure and a sealing method for an electronic component achieves desired sealing properties and heat resistance. An electronic component element is mounted on a substrate having an electrode pattern disposed thereon, and is sealed by bonding a cap for covering the electronic component element on the substrate. An adhesive structure used to join the cap on the substrate includes a high-glass-transition-point adhesive partially coated on the cap bonded portion of the substrate, and a low-glass-transition-point adhesive coated over an entire periphery of the opening of the cap. The opening of the cap is pressed on a mounting portion of the substrate, and the adhesives are then cured to bond and seal the substrate and the cap.

This is a Divisional of U.S. patent application Ser. No. 08/921,859filed on Aug. 25, 1997, now U.S. Pat. No. 5,886,457.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sealing structure and a method forsealing an electronic component, and more particularly, to a sealingstructure and a method of sealing a surface mount type electroniccomponent.

2. Description of the Related Art

A conventional electronic component known as a surface mount typeelectronic component, such as a piezoelectric oscillator, apiezoelectric filter, or the like, comprises a substrate having anelectrode pattern disposed on an upper surface thereof, a cap and apiezoelectric element disposed in a space defined between the substrateand the cap. An electrode of the piezoelectric element is connected andbonded to the electrode pattern disposed on the substrate via aconductive adhesive. The cap is placed on the substrate to cover thepiezoelectric element, and is bonded to the substrate by a sealingadhesive coated on the opening of the cap. The electrode pattern of thesubstrate extends outwardly from the bonded portion of the cap so thatan extended portion of the electrode pattern serves as an externalelectrode to define a surface mount type electronic component.

In such a surface mount type electronic component, when mounting failureoccurs after soldering and packaging with a printed substrate,corrective measures ranging from repair (removal) to re-soldering(manual soldering) are carried out in some cases. In re-soldering, aportion of the electronic component located near the electrode ismanually heated by a soldering bit, but heat is transmitted to portionsother than the electrode and creates a high-temperature portion.Generally, because a sealing adhesive is required to have flexibility, aresin adhesive having a relatively low glass transition temperature (Tg)is used. Transmission of heat having a temperature higher than the Tg ofthe sealing adhesive decreases adhesive strength of the sealingadhesive, and thus causes the problem of displacing or even removing thecap if an external force is applied.

To prevent this problem, a method using a sealing adhesive having a highTg has been considered. However, a very high Tg is required, and such asealing adhesive cannot provide other required physical properties,e.g., a desired expansion and contraction of a member in a heat shocktest, thereby causing a problem of fracture, peeling or cracks. Althougha high heat-resistant adhesive such as an inorganic adhesive can beused, such an adhesive cannot form a layer having a sufficient densenessto prevent penetration of a solvent during washing, and cannot provideand ensure good sealing properties.

SUMMARY OF THE INVENTION

To solve the problems described above, the preferred embodiments of thepresent invention provide a sealing structure and a method for sealingan electronic component which provide required sealing properties andrequired heat resistance.

According to a preferred embodiment of the present invention, there isprovided a sealing structure for an electronic component in which anelectronic component element is mounted on a substrate having anelectrode pattern disposed thereon, and a cap for covering theelectronic component element is bonded and sealed to the substrate, thesealing structure comprising a plurality of adhesives having differentglass transition points for bonding the cap and the substrate.

In the preferred embodiments of the present invention, the cap and thesubstrate are bonded and sealed with a first adhesive having high heatresistance and a high Tg and a second adhesive having excellent sealingproperties and a low Tg. When the electronic component is repaired andre-soldered after soldering and packaging with the printed substrate, insome cases, a soldering bit contacts the cap to heat the bonded portionof the cap to high temperature. However, because the first adhesivehaving a high Tg has high heat resistance, even if the bonded portion isheated to a high temperature, the adhesive strength of the sealingstructure experiences very little deterioration which is substantiallyless deterioration as compared to prior art sealing structures. As aresult, displacement and removal of the cap is prevented. Because thesealing properties in the heat shock test are ensured by the secondadhesive having a low Tg, the sealing properties do not deteriorate.

Although the coating position of each of the adhesives is not limited aslong as the sealing properties and adhesive force can be reliablyprovided, the cap and the substrate are preferably bonded and sealed bya method in which the first adhesive having high heat resistance and ahigh Tg is coated on the bonded portion between the cap and thesubstrate, and the second adhesive having excellent sealing propertiesand a low Tg is preferably coated over an entire periphery so as tocover the bonded portion. In this case, since the first adhesive havinga high Tg is partially coated, a gap for allowing entrance and exit ofair is defined when the cap is bonded.

In accordance with another preferred embodiment of the presentinvention, there is provided a method for sealing an electroniccomponent in which an electronic component element is mounted on asubstrate having an electrode pattern disposed thereon, and a cap forcovering the electronic component element is bonded and sealed to thesubstrate, the sealing method comprising the steps of preparing aplurality of adhesives having different glass transition points, coatinga first adhesive having a high glass transition point on a mountingportion of the substrate, coating a second adhesive having a low glasstransition point on an opening of the cap, pressing the opening of thecap on the mounting portion of the substrate, curing the second adhesivehaving a low glass transition point and curing the first adhesive havinga high glass transition point.

When the first adhesive having a high Tg is coated on a mounting portionof the substrate where the cap is bonded, and the second adhesive havinga low Tg is coated on the opening of the cap, the first adhesive and thesecond adhesive contact each other when the cap is mounted on thesubstrate. At this time, if the viscosity of the uncured first adhesiveis higher than that of the uncured second adhesive, the second adhesivehaving a low Tg is pushed away, and the opening of the cap is caused tobite into the first adhesive having the high Tg to temporarily fix thecap. The second adhesive having the low Tg spreads to surround the firstadhesive having a high Tg, thereby obtaining good sealing properties.

When both adhesives are heat curable adhesives, the low-Tg adhesive andthe high-Tg adhesive are preferably cured in the aforementioned order,but the low-Tg adhesive first decreases in viscosity during heating toform a fillet for wrapping the high-Tg adhesive therein, therebyobtaining more secure sealing properties.

When both adhesives are UV curable adhesives, the electronic componentneed not be heated, and thus it is possible to prevent the occurrence ofair cavities in the cap caused by expansion of air in the cap.

The adhesives may be a combination of a heat curable adhesive and a UVcurable adhesive.

For the first adhesive having a high Tg, an adhesive having a Tg higherthan an evaluation temperature, for example, the temperature of thebonded portion at the time of re-soldering, is preferable. For thesecond adhesive having a low Tg, an adhesive having a Tg lower than thesolder melting temperature may be used, but an adhesive having goodflexibility is preferable for forming a dense adhesive layer whichprevents penetration of a solvent during a washing step.

These and other elements, features, and advantages of the preferredembodiments of the present invention will be apparent from the followingdetailed description of the preferred embodiments of the presentinvention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view illustrating an electroniccomponent according to a preferred embodiment of the present invention;

FIG. 2 is a plan view illustrating the substrate shown in FIG. 1; and

FIGS. 3(a)-3(c) are sectional views taken along line B--B of FIG. 2,illustrating the steps of sealing an electronic component according topreferred embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a surface mount type piezoelectric oscillator A as oneexample of an electronic component according to a preferred embodimentof the present invention.

A substrate 1 preferably comprises a substantially rectangular sheetwhich is preferably formed by molding alumina ceramic and which has athickness of about, for example, 0.3 to 0.7 mm. Of course, the materialof the substrate 1 is not limited, and any materials such as dielectricceramics, glass epoxy, and other suitable materials can be used.

Electrodes 2 and 3 are disposed on the upper surface of the substrate 1and are preferably formed by a known method such as sputtering,evaporation, printing, plasma spraying or the like. In this preferredembodiment, to provide sufficient bonding strength and solderability,the electrodes 2 and 3 are preferably formed by printing Ag/Pt stovingtype conductive paste of 50 μm and then firing the paste. Both ends ofthe electrodes 2 and 3 preferably extend to concave through-holes orrecesses 1a respectively disposed at the ends of the long sides of thesubstrate 1, and connected to electrodes disposed on the lower surfaceof the substrate 1 through the electrodes disposed on the inner surfacesof the through-holes 1a.

An oscillator element 5 is bonded and fixed on the substrate 1 by amaterial 10 having conductivity and adhesion characteristics, such as aconductive adhesive. As the connecting material 10, for example, solderor a metallic terminal may be used as long as it has a functionequivalent to the conductive adhesive. The oscillator element 5 of thispreferred embodiment is preferably a thickness shear vibration modeoscillator in which a first electrode 7 extends over a region of about2/3 of the top surface of the piezoelectric ceramic substrate 6 from oneend thereof, and a second electrode 8 extends over a region of about 2/3on the surface from the other end thereof. However, other oscillatorsvibrating in other vibration modes may also be used for the oscillatorelement 5.

Ends of the electrodes 7 and 8 are disposed opposite to each other withthe piezoelectric substrate 6 located therebetween preferably atsubstantially the central portion of the substrate 6 to form a vibrationportion. In this preferred embodiment, the other ends of the electrodes7 and 8 extend to protective layers 9 disposed at both ends of thepiezoelectric substrate 6 and are connected to auxiliary electrodes 7aand 8a respectively disposed on the opposite surfaces throughend-surface electrodes 7b and 8b, respectively. These end-surfaceelectrodes 7b and 8b are respectively located between the protectivelayers 9 and the piezoelectric substrate 6.

Both ends of the oscillator element 5 are preferably bonded and fixed tothe substrate 1 by the conductive material 10 to electrically connectthe electrodes 2 and 3 of the substrate 1 and the electrodes 7 and 8 ofthe element 5. At this time, the electrode 8 on the bottom surface ofthe element 5 is connected to the electrode 2 without any material beingdisposed therebetween, but the electrode 7 on the upper surface isconnected to the electrode 3 through the auxiliary electrode 7a. Duringbonding, it is preferable to provide a slight vibration space betweenthe central portion of the element 5 and the substrate 1.

On the upper surface of the substrate 1, a frame-shaped insulating layer4 is preferably provided for securing insulation between the electrodes2 and 3 and a cap 11, which will be described below, and for flatteninga step formed by the electrodes 2 and 3. Although, in this preferredembodiment, the insulating layer 4 is preferably formed by baking glasspaste, the insulating material is not limited. When an insulatingmaterial is used on the cap 11, the insulating layer 4 is notnecessarily required.

The opening of the cap 11 is bonded to the insulating layer 4 of thesubstrate 1 so as to cover the element 5. Although ceramics such asalumina, resin, and metals can be used as the material for the cap 11,in this preferred embodiment, a metallic material press-molded in ashape having a substantially U-shaped cross section is preferably usedfor decreasing the size of the product and securing dimensionalprecision. Any metallic material can be selected as long as productstrength and adhesion can be obtained, and, for example, an aluminumalloy, nickel silver, 42 Ni alloy can be used.

An adhesive 12 having a high Tg is preferably partially coated on a capmounting and bonding portion of the substrate 1, particularly, on theinsulating layer 4. The adhesive 12 is preferably disposed at aplurality of positions by a method such as printing, pin transfer ordispersion. Although an epoxy adhesive, for example, having a Tg ofabout 198° C., may be used as the high-Tg adhesive 12, the type of theadhesive used is not limited as long as the adhesive 12 has a sufficientcapacity to adhere to a material to be coated and a sufficiently high Tg(150° C. or more) is provided. However, curing conditions are preferablymatched with the sealing adhesive 13 described below, or curing ispreferably stopped during the course of curing of the sealing adhesive13. Although the coating area and coating position of the adhesive 12are not limited, the adhesive 12 is preferably partially coated so as toform a gap for allowing entrance and exit of air when the cap 11 ismounted. In this preferred embodiment, the adhesive 12 is preferablycoated at a total of 4 positions on the longer sides of the frame-shapedinsulating layer 4. When the cap 11 is mounted on the substrate 11, thecap 11 is slightly pressed, for example, under about 300 gf/cap×0.2 sec,in order to cause the cap 11 to bite into the high-Tg adhesive 12.

The adhesive 12 is preferably simultaneously coated on a plurality ofportions on a mother substrate to decrease the number of the steps forcoating the adhesive 12 to only a few steps.

Along an entire periphery of the opening of the cap 11, a sealingadhesive 13 is coated by transfer, and the sealing adhesive 13 is bondedto the insulating layer 4 of the substrate 1. For the sealing adhesive13, a low-Tg adhesive, e.g. Tg is equal to about 50 to about 100° C.,having flexibility is preferably used. In this preferred embodiment, anepoxy type heat curable adhesive having a Tg equal to about 80° C. wasused, but an epoxy acrylate UV curable adhesive having a Tg equal toabout 60° C. has the same function.

After the cap 11 is press-bonded to the substrate 1, the piezoelectricoscillator A is introduced into a curing furnace and then heated in apredetermined temperature profile to cure the adhesives 12 and 13. Inheat treatment, since the air in the cap 11 escapes due to expansion byheating, and the cap 11 is moved due to the reaction of the escape ofair, the cap 11 is preferably pressed by a jig during curing. When a UVadhesive is used, the adhesive may be cured by irradiation of apredetermined quantity of light in a UV furnace. In this case, unlikeheat treatment, there is no respiration, and thus the cap 11 need not bepressed by a jig.

FIGS. 3(a)-3(c) shows a sealing method in another preferred embodimentof the present invention.

FIG. 3(a) shows the state before the cap 11 is mounted in which theinsulating layer 4 is disposed on the cap mounting and bonding portionof the substrate 1, and the high-Tg adhesive 12 is partially coated onthe insulating layer 4. On the other hand, the low-Tg adhesive 13 ispreferably coated over the whole periphery of the opening of the cap 11.

FIG. 3(b) shows a state where the cap 11 is mounted. The high-Tgadhesive 12 previously coated on the substrate 1 rises in a step formdue to the thickness thereof. When the cap 11 is press-mounted on theadhesive 12, the low-Tg adhesive 13 coated on the opening of the cap 11is pushed away toward both sides, and the opening of the cap 11 bitesinto the high-Tg adhesive 12. At this time, the viscosity of the uncuredhigh-Tg adhesive 12 is lower than that of the uncured low-Tg adhesive13. As the difference between the viscosities of the uncured adhesives12, 13 increases, the adhesive 12 is more securely replaced by theadhesive 13. In this preferred embodiment, the viscosities of theuncured high-Tg adhesive 12 and the uncured low-Tg adhesive 13 are about15,000 cps and 28,000 cps, respectively (25° C., 5 rpm).

FIG. 3(c) shows the state after the electronic component A is heated. Inheating, the adhesives 12 and 13 are heat-cured in the order of thelow-glass-transition-point adhesive and the high-glass-transition-pointadhesive, and thus the low-Tg adhesive 13 is first decreased inviscosity to form a fillet 13a outside and inside the cap 11 so as towrap the high-Tg adhesive 12 therein. Therefore, good sealing propertiesare reliably and securely be obtained. When a UV-curable adhesive isused as the low-Tg adhesive 13, an adhesive having proper viscosity atroom temperature is selected so that the fillet 13a can be formed.

When mounting failure is found after the electronic component formed asdescribed above is solder-mounted on the printed substrate, theelectronic component is sometimes repaired and re-soldered. Inre-soldering, a soldering bit contacts the cap 11 which usually causesdeterioration of the sealing properties of the adhesives used to bondthe cap to the substrate. However, even if the cap 11 is heated to hightemperature, because a sufficiently large adhesive strength is securedby the high-Tg adhesive 12, the cap 11 can be prevented from beingshifted or removed. Thus, the sealing properties can be secured by thelow-Tg adhesive 13.

The present invention can be applied not only to a piezoelectricoscillator containing an oscillator element in the above-describedpreferred embodiment, but also to an oscillator with built-incapacitance comprising an oscillator element and a capacitor element,and an oscillator with built-in capacitance comprising a capacitorportion formed on a substrate. The present invention can also be appliedto other electronic components such as a filter, a circuit module, etc.

As can be seen from the above description, the preferred embodiments ofthe present invention use a high-Tg adhesive and a low-Tg adhesive asadhesives for bonding a substrate and a cap, and thus, superior sealingproperties are achieved by the low-Tg adhesive in a heat shock test, andshift or removal of the cap is prevented by the high-Tg adhesive even ifthe temperature is elevated due to contact with a solder bit or thelike. Therefore, an electronic component having excellent heatresistance is obtained.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A method for sealing an electronic component, theelectronic component including an electronic component element mountedon a substrate and a cap covering the electronic component element beingbonded and sealed to the substrate, the method comprising the stepsof:preparing at least two adhesives having different glass transitionpoints; coating a first of the least two adhesives having a high glasstransition point on a portion of the substrate; coating a second of theat least two adhesives having a low glass transition point on a portionof the cap; mounting the cap on to the substrate; and curing the firstand second adhesives.
 2. A method for sealing an electronic componentaccording to claim 1, wherein the step of curing the first and secondadhesives includes the step of first curing the second adhesive having alow glass transition point and then curing the first adhesive having ahigh glass transition point.
 3. A method for sealing an electroniccomponent according to claim 1, wherein the first adhesive having a highglass transition point is partially coated on a cap mounting portion ofthe substrate, and the second adhesive having a low glass transitionpoint is coated over an entire peripheral rim of an opening of the cap.4. A method for sealing an electronic component according to claim 1,wherein a viscosity of the first adhesive having a high glass transitionpoint when in an uncured state is higher than that of the secondadhesive having a low glass transition point in an uncured state so thatwhen the opening of the cap is mounted on the substrate, a rim of thecap is caused to bite into the first adhesive having a high glasstransition point.
 5. A method for sealing an electronic componentaccording to claim 1, wherein at least one of the first and secondadhesives is a heat curable adhesive.
 6. A method for sealing anelectronic component according to claim 1, wherein at least one of thefirst and second adhesives is a UV curable adhesive.
 7. A methodaccording to claim 1, wherein the at least two adhesives are arranged incontact with each other.
 8. A method according to claim 1, furthercomprising the step of disposing a frame-shaped insulating layer betweenthe substrate and the electronic component element.
 9. A methodaccording to claim 1, wherein each of the first and second adhesives aredisposed between the substrate and the cap.